Using15N, the fate of N applied to wetland rice either asAzollaor urea was studied in a field at the International Rice Research Institute (IRRI). In bigger plots nearby, yield response and N uptake were also determined with unlabelled N sources.Azollamicrophylla was labelled by repeated application of labelled ammonium sulfate. Labelled and unlabelled N were used alternately in applications ofAzollaor urea 0 and 42 days after transplanting, in order to determine the effect of the time of application on the availability ofAzollaN. The quantities ofAzollaN incorporated were 23 more than those of urea N (30 kg N ha−1) in the isotope plots or 7 less in the yield response plots. Grain yield and total N uptake by the rice plants in the yield-response plots were higher in the urea-treated plots than in theAzolla-treated plots, but the physiological effect ofAzollaN (grain yield response/increase in N uptake) was higher than that of rea. The labelled N balance was studied after the first and second crops of rice. Losses of labelled N after the first crop were higher from urea (30–32) than fromAzolla(0–11 ). Losses in N applied as a side dressing 42 days after transplanting were less than those of N applied basally. No further losses of15N occurred after the first crop. The recovery ofAzolla15N in the first crop of rice was 39 from the basal application and 63 from the side dressing. The recovery of urea15N was 27 from the basal application and 48 from the side dressing. Recoveries of residual N from bothAzollaand urea during the second rice crop were similar. Laboratory incubation of theAzollaused and the changes in labelled exchangeable N in the soil showed that at least 65 ofAzollaN (4.7 N content) was mineralized within 10
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